The assessment of climate change dictates a global perspective and a very long
time horizon that covers periods of at least a century. As the prediction of
future anthropogenic GHG emissions is impossible, alternative GHG emissions
scenarios become a major tool for the analysis of potential long-range developments
of the socio-economic system and corresponding emission sources.

However, to develop scenarios for a period of 100 years is a relatively new
field. Difficulties arise not only from large scientific uncertainties and data
inadequacies, but also because people are not trained to think in such time-spans.
We are educated in narrow disciplines, and our ability to model complex systems,
at the global level, is still in its infancy. For example, within the next century
technological discontinuities should be expected, and possibly major shifts
in societal values and in the balance of geopolitical power. The study of past
trends over such long periods is hampered because most databases are incomplete
if more than 50 years old. Given these gaps in our data, methods and understanding,
scenarios are the best way to integrate our demographic, economic, societal
and technological knowledge with our understanding of ecologic systems to evaluate
sources and sinks of GHG emissions. Scenarios as an integration tool in the
assessment of climate change allow a role for intuition, analysis and synthesis,
and thus we turn to scenarios to take advantage of these features and aid the
assessment of future climate change, impacts, vulnerabilities, adaptation and
mitigation. Since the scenarios focus on the century time scale, tools are used
that have been developed for this purpose. These tools are less suitable for
the analysis of near-term developments, so this report does not intend to provide
reliable projections for the near term.

To provide input for evaluating climatic and environmental consequences
of alternative future GHG emissions in the absence of specific measures to
reduce such emissions or enhance GHG sinks.

To provide similar input for cases with specific alternative policy interventions
to reduce GHG emissions and enhance sinks.

To provide input for assessing mitigation and adaptation possibilities,
and their costs, in different regions and economic sectors.

To provide input to negotiations of possible agreements to reduce GHG emissions.

The SRES emissions scenarios are intended for the first, third and fourth uses.
They do not include any additional (explicit) policies or measures directed
at reducing GHG sources and enhancing sinks. Thus, they cannot be directly applied
to the second purpose of emissions scenarios. Instead, they could be used as
reference cases for the introduction of specific policy interventions and measures
in new model runs that share the same specifications for the other principal
driving forces of future emissions. However, the SRES emissions scenarios include
a host of other policies and measures that are not directed at reducing sources
and increasing sinks of GHGs, but that nevertheless have an indirect effect
on future emissions. For example, policies directed at achieving greater environmental
protection may also lead to lower emissions of GHGs. Moreover, afforestation
and reforestation measures increase CO2 sinks, and a shift to renewable energy
sources reduces the sources of emissions.

Within three of the broad objectives listed above, the new SRES emissions scenarios
are also intended to meet the specific needs of three main IPCC user communities:

Working Group I (WGI), which includes climate modelers who need future emission
trajectories for GHGs and aerosol precursors as inputs for the GCMs used to
develop climate change scenarios.

Working Group II (WGII), which analyzes climate impacts and adaptation policies,
first need the climate-change scenarios produced by WGI's climate modelers.
Second, analysts need to know the socio-economic changes associated with specific
emissions scenarios, as impacts of climate change on ecosystems and people
depend on many factors. Among these are whether the people are numerous or
few, rich or poor, free to move or relatively immobile, and included or excluded
from world markets in technologies, food, etc.

WGIII, which analyzes potential mitigation policies for climate change,
also needs to know the socio-economic settings against which policy options
are to be evaluated. Are markets open or protected? Are technological options
and economic resources plentiful or scarce? Are people vulnerable or adaptable?

The interests of these three user groups create certain requirements that the
SRES scenarios attempt to fulfill. For example, climate modelers and those who
analyze climate impacts need scenarios on the order of 100 years because of
the long response time of the climate system. At the same time adaptation-policy
analysis tends to be focused more on the medium-term, around 20 to 50 years.
The SRES scenarios attempt to include enough information and specific details
to be useful to these groups. Spatially explicit emissions and socio-economic
variables are required for slightly different reasons. Some emissions, such
as the SO2 emissions that contribute to sulfate aerosols, have impacts that
vary depending on where they are emitted. Climate modelers therefore need spatially
explicit emission estimates. Similarly, impacts depend on the geographic patterns
of changing temperatures, rainfall, humidity and cloud cover, and how these
compare to evolving socio-economic patterns in specific scenarios. Impact modelers
therefore need spatially explicit estimates of, in particular, population growth,
migration, and the economic variables that reflect the expected adaptability
or vulnerability of different populations and regional economies to different
regional climate changes.

Taking the above audiences and purposes into account, the following more precise
specifications for the new SRES scenarios were developed. The new scenarios
should:

cover the full range of radiatively important gases, which include direct
and indirect GHGs and SO2 ;

have sufficient spatial resolution to allow regional assessments of climate
change in the global context;